Making dolomite briquettes for use in the production of magnesium



Patented May 5, 1942 7 'UNlTED STATES PATENT OFFICE MAKING DOLOMIT EBRIQUETTES FOR USE IN THE PRODUCTION OF MAGNESIUM John s. Peake,Midland, Mich, assignor to The Do'w Chemical Company, Midland, Mich., acorporation of Michigan No Drawing. Application May 26, 1939, Serial No.276,006

6 Claims. (01. 75-67) This invention relates to a method for thecountered during the thermal reduction operaproduction 01' strongabrasion resistant britions without powdering or crumbling. Furtherquettes containing a magnesium carbonate or .objects will becomeapparent from the followoxide and a reducing agent, for use in thepreping description.

aration of metallic magnesium. According to the present invention, amechan- In the electrothermic reduction of dolomite icallystrongbriquette, whereby the aforesaid or of magnesite by means of reducingagents object may be attained, can be produced by desuch as aluminum,silicon, or the equivalent gassing dolomite or equivalent material, suchferrosilicon, the mixture of reagents has been as a mixture of magnesiteand quick-lime, toemployed in powdered and in briquette form. 10 getherwith the reducing agent to be employed, It has been observed that thepowdered charge at a temperature up to 1000 C., and preferably tends tocause difliculty during introduction of under reduced pressure, toremove carbon dithe charge continuously into'the reduction fu'roxide,water, and compounds capable of producnace. Under the reduced pressurecustomarily I ing the same when heated. The so-treated comemployed inthe reaction zone, powdered 'maposition is then compressed intobriquettes of terial is too readily carried over to the consuitable sizeand shape under pressures in the denser, contaminating the distilledmagnesium. range from about 2700 to 5000 pounds per The briquettesheretofore employed have not square inch. In order to avoiddisintegration been sufilciently hard to resist abrasion as they of thebriquettes as a result of moisture absorpcome in contact with oneanother and with the n, they a imm y placed in a muffle walls-of thevessels in which they are handled. furnace 'or other suitable retort andheated to Even when such high pressures as 15,000 to about 1000 0.,whereby the hardening of the 30,000 pounds per square inch are employedin composition is efiected. After the briquettes forming briquettes fromthe customary powhave been hardened and cooled to room temperderedmixture of dolomite and reducing agent, ature, they may be stored inclosed cans or in the shaped pieces lack suiiicient coherence to adesiccator without deterioration, retaining prevent powderlng due toattrition, and the nattheir characteristic hardness. In practice, itural consequence is that in the thermal reduchas been customary o mp therdened tion zone suflicient powder is rubbed oil from q ettes in theproduction of metallic magthe brlquette either to form solid deposits onnesium soon after they have been subjected to the walls of the furnacechamber or to contamthe h r ni peration. They may even be inate themetal in the condensing zone after transferred out of access to coldair' directly distillation from the hardening zone to the reduction zoneMany of the difiiculties heretofore encoun- Without cooling. tered inpreparing briquettes from dolomite or In a preferred manner of carryingout the like charges ior magnesium production furnaces n n, calcineddolomite or an equivalent have arisen from the fact that the powderedSource of magnesium is crushed and ground to calcined dolomite containstoo much moisture & Suitable de ree of fineness, and preferably to andcarbon dioxide or compounds which yield about 200 350 mesh The reducingagent isthese substances when heated. The result of 40' crushed andground to a similar state of subthe presence of these undesirable agentsis that division. The finely divided magnesium yieldthe briquettes crackand disintegrate at reducing substances and the finely divided reducingtion temperatures, and give lowered yields of a ent are mixed and themixture heated prefermagnesium. This latter difliculty is particularly yn an nert a mosphere or in vacuum at apparent in the use of briquettesmade using temperatures from about 650 to 10000 C. and water as abinder, as has been proposed in the preferably near 800 C. The timrequir d may vary y. from a few minutes to several It is accordingly anobject of the present inhours, depending on the equipment and maventionto provide a mechanically strong briterials employed. The degassedcharge i cooled quette of dolomite or of a mixture of in'agnesite toroom temperature and the finely divided powand quick-lime, or equivalentmagnesium source, der compressed in a mold of the desired Shape with areducing agent such as aluminum, ierroand size to produce briquettes,suitabl pr silicon or silicon, which briquette can be probeing in therange. from 2700 to 5000 pound per duced under reasonable conditions ofpressure square inch, and preferably about 3500 to 4000 and which willwithstand the abrasive forces enpounds per square inch. At this stage inoperstrength and crumble easily when subjected to As soon as practicaland before attrition. enough time has elapsed to cause disintegration,the briquettes are placed in a muflle, or equivalent furnace and heatedto a hardening temperature below that at which thermal reductioncommences, suitably to about 1000' C. If desired, heating may be gradualwith, for example, 200 centigrade degrees rise in temperature per houruntil the furnace temperature has reached about 100.0" C. This lattertemperature is'maintained for about 1 hour, more or less, whereafter'the muflie is allowed to cool slowly to room temperature. The resultingbriquettes, as previously stated, may be stored in desiccators withoutlosing their strength characteristics or they may be employedimmediately for the preparation of metallic magnesium by known reductionprocesses. The hardening operation,

.at about 1000 C., may be carried out, especially when large briquettesare being produced, in an atmosphere of air if desired, or in the caseof small briquettes, the high temperature'zone may be filled with aninert gas such as hydrogen or the heat treatment may be carried outunder conditions of high vacuum.

Tests have shown that, while non-carbonaceous binders, such assilicates, may be employed if desired, no material advantage is to beobtained by employing binders for the briquette material as theabove-described method produces a briquette without the use of a binderwhich has all of the strength characteristics now deemed desirable.Small amounts of various compounds which will be of use in the thermalreduction zone'wherein metallic magnesium is prepared may, if desired,be incorporated in the mix from which the briquettes are prepared. Caremust be taken to see that such addition'agents are freed from moisture,carbon dioxide, or compounds capable of producing gases under theconditions employed in the reduction 'step. Among such compounds whichmay be added in small amount with advantage may be named fluorspar whichis customarily employed as a catalyst in the reducing operation. It isto be understood that a mixture of mag-'- nesium oxide and lime ormagnesite and quickline is, for the present purpose, equivalent todolomite and that a calcined mixture of these materials subjected to thetreatment previously outlined can be briquetted with a reducing agent toproduce an article of sufficiently high mechanical strength to avoid thedifficulties heretofore encountered. 1

The following example illustrates the practice of the invention:

. Example 100 parts of calcined dolomite and 20 parts by weight of '75per cent ferrosilicon, each having a particle size of about 200 mesh,were mixed and degassed by heating for 20 hours at 700 C. under a highvacuum. The powdered material was compressed into briquettes about 3inches thick and 3.75 inches in diameter employing a pressure of 3650pounds per square inch. Compression was applied over a period of 45seconds between the time the piston first encountered the powderedmaterial in the mold and the time of pressure release. The so-formedbriquettes were heated as soon as practical and before enough time hadelapsed to' cause disintegration in a muffle furnace in an atmosphereofair with a temperature rise of 200 centigrade degrees per hour untilthe temperature reached 1000 C. The latter temperature was maintainedfor about 1 hour, after which the heat was turned off and the briquetteswere allowed to cool gradually to room temperature in the closedfurnace. The resultingbriquettes were very hard and abrasion resistantunder the conditions employed in the pressures averaging 335 pounds persquare inch were applied. The weakest of the baked briquettes wascrushed under a load of over 250 pounds per square inch. Several twoyear old baked briquettes withstood loads near 300 pounds per squareinch before breaking. By way of comparison, several unbaked briquettesof the same composition withstood average crushing loads of only 121pounds per square inch, the strongest unbaked briquette being crushedwhen a load of 158 pounds per square inch was applied. The increasedstrength of .the briquettes made according to the invention is evidenceof their greater utility as compared with the briquettes of the priorart. No crumbling or powdering is encountered in operation when usingthe new briquettes in the manufacture of magnesium.

The mixes employed in the preparation of the herein-described briquettesmay have any of the customary proportions of magnesium-containingmaterial and reducing agent, the said proportions varying with theconcentration of magnesium oxide in the mineral employed and with theparticular reducing agent selected. To illustrate, parts of a typicaldolomite containing about invention may be employed instead of thoseexplained, change being made as regards the method herein disclosed,provided the step or steps stated by any of the following claims or theequivalent of such stated step or steps be em-' ployed.,

I therefore particularly point out and distinctly claim as my invention:

1. The method which comprises subjecting a calcined mixture of amagnesiumcontaining compound selected from the class consisting ofmagnesium carbonate and magnesium oxide and 7 mixtures containing thesame which are capable of being reducedto form magnesium in finelydivided form together with a finely divided reducing agent for the saidmagnesium-containing material to a degassing temperature up to about1000 C. for a period sufiicient to remove from the said mixturemoisture, carbon dioxide,

and other materials capable of yielding gases under the conditions ofthermal reduction of ma nesium-containing compounds, thereaftercompressing the said finely divided mixture intobriquettes and thenheating the so-fornied briquettes to a hardening temperature below thatat which thermal reduction commences, thereby to produce briquettes ofimproved mechanical strength and hardness.

2. The method according to claim 1, wherein the initial finely dividedmix is or a particle size between about 200 and about 350mesh.

3. The method according to claim 1, wherein the degassing operation isconducted in an inert atmosphere at a temperature between 650"v and 1000C.

4. The method which comprises mixing calcined dolomite and silicon orierrosilicon, both of approximately 200 to 350 mesh particle size, inproportion suitable for employment in the thermal reduction of dolomiteto produce metallic magnesium, subjecting the so-formed powdered mixtureto a degassing temperature in the range from about 650 to 1000 C. in aninert atmosphere tor a period sufiicient to remove from the said mixturemoisture,- carbon dioxide, and other materials capable of yielding gasesunder the conditions of thermal reduction of magnesium-containingcompounds, thereafter compressing the said finely divided mixture underpressuresin the rangefrom 2700 to 5000 pounds per square inch, andimmediately thereafter heating the so-formed vbriquettes gradually to atemperature near 1000 0., thereby to produce briquettes of improvedmechanical strength and hardness.

5. The method according to claim 1, wherein the briquettes are heatedwith a gradual temperature rise of about 200 centigrade degrees per houruntil the temperature is about 1000 C., the latter temperature beingmaintained for about an hour, whereupon the briquettes are allowed tocool gradually in the closed furnace to room temperature.

6. The method according to claim 4, wherein the briquettes are heatedwith a gradual temperature rise of about 200 centigrade degrees per houruntil the temperature is about 1000 C., the latter temperature beingmaintained for about an hour, whereupon the briquettes are allowed tocool gradually in the closed furnace to room temperature.

' JOHN S. PEAKE.

